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PUE stands for power usage effectiveness. It’s no secret that data centers are major consumers of energy and that their IT infrastructure is made up of some particularly power-hungry pieces of equipment. By understanding and tracking data center PUE, managers can begin to understand how efficient the IT infrastructure is and if it is becoming more or less efficient over time.

How to Calculate Data Center PUE

Data center PUE is the ratio of total facility energy to IT equipment energy used in a data center. It can be calculated by dividing total facility energy usage by IT equipment energy usage. 

Total facility energy encompasses the power dedicated to the data center facility or data room, measured at the meter. This includes all loads, including IT equipment, cooling systems, lighting systems, and power delivery components. 

Total IT equipment encompasses all energy fed to compute, storage, and networking equipment, including other control equipment like KVM switches, workstations, monitors, and laptops. 

When the two numbers are divided, a decreasing quotient shows that overall data center sustainability is improving as the IT infrastructure becomes more efficient.

Why the PUE Data Center Metric Can Be Tricky

Despite the simplicity of the ratio and the acceptance as a standard performance metric, calculating PUE is not as straightforward as the formula makes it seem. There are several practical considerations.

  • Generally, data center PUE is not a one-time measurement. Tracking data center PUE over time for a single data center will reveal the facility's performance against its initial baseline calculation. Although Uptime Institute reports the average PUE in data centers for 2020 as 1.58, this metric may not be entirely useful for comparison across multiple data centers, each with unique configurations. 
  • Comparing PUE measurements against various data centers, even very similar facilities, proves to be challenging. Two data centers, roughly the same size but in different locations (perhaps different regions or countries), could easily consume power in dramatically different ways for a number of reasons. For instance, differences in local climate and weather, power grid services, and even facility building materials may impact energy usage and data center energy efficiency. 
  • What is considered material in the PUE data center definition differs by data center. Different data center teams do not always have the same ideas about what is relevant when it comes to calculating data center PUE. Teams need to consider how to classify subsystems as IT loads, infrastructure loads, or irrelevant, and even consider if a measurement is practically possible. Potential challenges for comparing data center PUE and energy efficiency across facilities include: 
    • Subsystems present in one data center may not be in another
    • Subsystems support mixed-use facilities, such as non-data center functions like cooling towers or chillers, and cannot be easily or directly measured
    • Instrumenting specific subsystems is impractical and costly, like power distribution units (PDUs), which contain many outlets multiplied by the number of PDUs in use in the facility
    • Practical measurement points, such as the meter, may include loads unrelated to the data center and prove to be difficult to separate

3 Ways to More Accurately Calculate PUE and Data Center Energy Efficiency

These tips can help lead you to a more reliable PUE:

1. Be aware of where PUE measurements are taken. Where energy measurements are taken impacts how to attribute power usage to its appropriate category, either facility usage or IT usage. For facility readings, take measurements at or near the facility's utility meter. For mixed-use facilities or data centers, target only the meter that powers the data center.

2. Account for shared utility meters. Data centers that share a utility meter with non-data center portions of a facility pose problems for attributing accurate measurements. In these cases, estimating the non-data center portion and deducting it from total usage may be the only option.

3. Get the timing right. IT equipment load should be measured after all power conversions, switching, and conditions have been applied. Within server rooms, PDUs are the most accurate site for measuring the total power delivered to the server racks.

Read the following blog to learn more: Choosing the Right Rack PDU Based on Power Requirements

How to Improve Data Center Efficiency and Create a More Sustainable Data Center

PUE is most valuable when it comes to benchmarking data center energy efficiency. If the measurement process is automated through energy performance software and in real-time, PUE data can provide a level of insight with significant business applications. For example, by understanding power consumption at various granular levels of the data center, power leaks and under-utilized systems can easily be discovered, and their costs curtailed.

Other Data Center Sustainability Metrics

In addition to PUE, other effectiveness-style metrics from The Green Grid have surfaced as usable measures for data center energy efficiency.

  • Data center infrastructure efficiency (DCiE) is the reciprocal of PUE. It measures the energy efficiency of a data center. DCiE can be calculated by dividing IT equipment energy usage by total facility energy usage. It can also be calculated by dividing 1 by PUE.
  • Energy reuse effectiveness (ERE) measures a facility's effectiveness in redirecting energy to other uses in the facility, such as using a heat exchanger to convert server discharge heat into heating for surrounding buildings.
  • Water usage effectiveness (WUE) attempts to measure the amount of water used by data centers to cool IT assets. 
  • Carbon usage effectiveness (CUE) is the ratio of the total carbon dioxide (CO2) emissions caused by total data center energy consumption to the energy consumption of IT equipment.

Read the following white paper to learn more: Calculating the Impact of Water Usage on Data Center Costs and Sustainability

Buying the Right UPS in Support of Data Center PUE

For comprehensive direction on purchasing uninterruptible power supplies, visit Vertiv's UPS Buying Guide. In the guide, learn to assess power needs by asking the right questions and taking other technical considerations into account before investing in your critical power infrastructure.

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